Mechanisms for the selective expression of a restricted fraction of a genome are inherent in the structure of that genome. Our studies focus on the assembly of chromatin and the ways in which the information for the control of differential gene expression may be transmitted across cell generations. Experiments using cytosine arabinoside to uncouple histone and DNA synthesis have allowed us to distinguish those features of normal chromatin structure which rely on a correct histone: DNA stoichiometry and those which are independent of the amount of available histone. As well, we have begun to define certain nonhistone-chromatin complexes of in vivo significance. Our future work will continue to focus on the replicative behavior of these nonhistones and their interaction with transcribable gene sequences.